This invention relates to the formation of a protective skin on blocks of microporous thermal insulation material, particularly to such blocks having complicated shapes and which have surfaces which are not to be protected in use.
A microporous thermal insulation is known which comprises silica-aerogel particles, an opacifier and fibre reinforcement, in a dry mix which is compacted to form a handleable block of material. Such materials and methods for its manufacture are disclosed in British Pat. Nos.: 1,205,572; 1,433,478; and 1,580,909, to which reference is directed. These materials are marketed by Micropore International Limited and its associated companies under the Trade Name MICROTHERM. Typically, the density of a block of MICROTHERM material is around 240 kg/m.sup.3. This material has excellent insulation properties but is of a powdery consistency and as a consequence, is often used with a cover layer of for example, fibreglass, to protect it from damage during normal handling. For some purposes, the material can be compressed in a porous envelope as described in British Pat. No.: 1,247,674. A material having satisfactory properties for some applications can consist only of the silica-aerogel particles, or of the particles and one of the opacifier and the reinforcement. The term silica-aerogel particles is deemed to include microporous open lattice silica structures generally having a small particle size of the order of 100 millimicrometers or less.
For a number of applications, it is not possible to form a block confined in an envelope, particularly where the requisite shape is complicated and, while flat surfaces can be easily covered, curved or undulating surfaces cause some difficulty. Complicated shapes are sometimes required to accommodate ducting or electrical connections to and from the region which is to be isolated. Adhesive coatings can be used or strips of material otherwise adhered but, because of the inherent dampness of many adhesives, the adjacent insulation material can be broken down by the moisture content and the bond substantially weakened. Satisfactory application of such coatings is time-consuming and costly and the difficult nature of the operation means that very close tolerances cannot always be met and some shapes cannot be properly protected.
Another thermal insulation material is known which has a similar composition to that described above but includes a binder for the components of the mixture. While as a consequence, the consistency of a block formed therefrom is less powdery, because the quantity of binder is always kept to a minimum to minimise the loss of insulating properties, similar problems to those set out above do arise.